The http://en.wikipedia.org/wiki/Z%C3%B3calo is the city center (of Mexico City's historical district), a simple and untaken metaphor for a service that "...provides users with a central location for both the documents and files they are...".
It's a shame Amazon can't afford to hire a marketing genius like you, but I think they're doing the best they can with the idiots they have to work with.
"Parse"? OK, sure...but that's not any more functionally descriptive than "Cognito"...and Parse is a more narrowly focused company with a primary service...In the case of "Cognito" and "Zocalo"..., Amazon has to come up with names that are distinctive because they provide so many core services...so many that even me, an engineer, get them confused. Imagine the difficulty of a fly-by marketing person trying to differentiate between similar sounding names.
Furthermore, these names are distinctive industry wide...presumably, if Zocalo becomes big, you will no longer have to say "AWS Zocalo", but just, "Zocalo"...just like you do with "S3" and "EC2".
To drive home the situation that Amazon is in, let's consider another great product name: Dropbox. Unlike Parse, it is very easy to relate to the core service. But consider the phrase, "Amazon Dropbox"..."Dropbox" is unique compared to the currently named AWS services...but it has a huge collision with Amazon's core service, which is, stocking and shipping products. If Dropbox.com didn't exist, and Amazon used it instead of "Zocalo"...there is a considerable risk that people associate "dropbox" with some kind of packaging thing.